Microelectronic devices are used in cell phones, pagers, personal digital assistants, computers, and many other products. A packaged microelectronic device can include a microelectronic die, an interposer substrate or lead frame attached to the die, and a molded casing around the die. The microelectronic die generally has an integrated circuit and a plurality of bond-pads coupled to the integrated circuit. The bond-pads are coupled to terminals on the interposer substrate or lead frame. The interposer substrate can also include ball-pads coupled to the terminals by traces in a dielectric material. An array of solder balls is configured so that each solder ball contacts a corresponding ball-pad to define a “ball-grid array.” Packaged microelectronic devices with ball-grid arrays are generally higher grade packages that have lower profiles and higher pin counts than conventional packages that use a lead frame.
Packaged microelectronic devices are typically made by (a) forming a plurality of dies on a semiconductor wafer, (b) cutting the wafer to singulate the dies, (c) attaching individual dies to an interposer substrate, (d) wire-bonding the bond-pads of the dies to the terminals of the interposer substrate, and (e) encapsulating the dies with a molding compound. It is, however, time consuming and expensive to mount individual dies to interposer substrates or lead frames. Also, as the demand for higher pin counts and smaller packages increases, it becomes more difficult to form robust wire-bonds that can withstand the forces involved in molding processes. Moreover, the handling processes for attaching individual dies to interposer substrates or lead frames may damage the bare dies. Packaging processes, therefore, have become a significant factor in producing semiconductor devices and other types of microelectronic devices.
Another process for packaging microelectronic devices is wafer-level packaging. In wafer-level packaging, a plurality of microelectronic dies are formed on a wafer, and then a redistribution layer is formed over the dies. The redistribution layer has a dielectric layer, a plurality of ball-pad arrays on the dielectric layer, and conductive traces in the dielectric layer that are coupled to individual ball-pads of the ball-pad arrays. Each ball-pad array is arranged over a corresponding die, and the ball-pads in each array are coupled to corresponding bond-pads of the corresponding die by conductive traces. After forming the redistribution layer on the wafer, a highly accurate stenciling machine deposits discrete masses of solder paste onto the individual ball-pads. The solder paste is then reflowed to form small solder balls or “solder bumps” on the ball-pads. After forming the solder balls, the wafer is singulated to separate the individual microelectronic devices from one another.
Wafer-level packaging is a promising development for increasing efficiency and reducing the cost of manufacturing microelectronic devices. By “pre-packaging” individual dies with a redistribution layer before cutting the wafers to singulate the dies, sophisticated semiconductor processing techniques can be used to form smaller arrays of solder balls. Additionally, wafer-level packaging is an efficient process that simultaneously packages a plurality of dies, thereby reducing costs and increasing throughput.
Packaged microelectronic devices can also be produced by “build-up” packaging. For example, a sacrificial substrate can be attached to a panel that includes a plurality of microelectronic dies and an organic filler that couples the dies together. The sacrificial substrate is generally a ceramic disc and it is attached to the active side of the dies. Next, the back side of the dies is thinned and a ceramic layer is attached to the back side. The sacrificial substrate is then removed from the active side of the dies and build-up layers or a redistribution layer is formed on the active side of the dies.
The build-up packaging process, however, has several drawbacks. For example, the build-up process is relatively expensive and may not be used on equipment set up for circular substrates. Furthermore, the resulting packaged microelectronic devices may not be stacked on top of each other to reduce the surface area or “footprint” of the devices on a printed circuit board. Accordingly, there is a need for an efficient and cost-effective process to package microelectronic devices that are stackable.